1. Field of the Invention
The present invention generally relates to signalling over fiber optic communication links and, more particularly, to devices for coupling a signal from one optical fiber to one or more other optical fibers and techniques for fabricating the same.
2. Description of the Prior Art
Conduction and guidance of light through the use of optical fibers is currently well-known and structures for optical fibers capable of conducting light over substantial distances with little light loss are relatively well-developed. More recently, the light guiding and conducting properties of optical fibers have been used as an alternative medium for signal communications where signals were traditionally conducted by electrical signalling.
As compared with signalling over wired circuits or wireless links such as radio or microwave communications, fiber optical communication links offer several distinct advantages. For example, communication using modulated light over fiber optics, while potentially subject to interference from stray light, is, in general, much more easily protected from environmental noise. In contrast, shielding electrical communication links from inductively or capacitively coupled noise may be extremely difficult compared to the inherent rejection of ambient optical noise by fibers of the total internal reflection (TIR) type now in widespread use.
Further, while neither light conducting fibers nor electrically conductive wiring are ideal conductors of their respective media, an ideal conductor of light can be much more closely approached with optical fibers of relatively common materials. For example, glass fibers having layers of different refractive indices, can yield a close approach to total internal reflection while losses associated with an electrical conductor may be significant, even when using relatively rare materials such as gold for the conductor.
However, some difficulties are encountered in optical communication links, particularly as such links become longer and more complex. These difficulties may be readily appreciated by recognizing that any optical boundary or junction, including geometrical variation represent imperfections in transmissivity which are relatively large in comparison with their electrical counterparts and particularly in comparison with the performance of uninterrupted optical fibers. For example, a splice from one length of optical fiber to another implies at least two boundaries, each of which will necessarily be partially transmissive and partially reflective due to differences in refractive indices at these boundaries. Various solutions to the imperfections necessarily caused by such commonly required practical fittings have, in many instances yielded substantial success.
For example, gels or oils having indices of refraction matched to optical fiber materials, together with relatively simple grooved alignment devices yield adequate transmissive splices. Splices may also be done by fusing the ends of optical fibers to remove the boundaries. However, some geometrical error or variation is substantially unavoidable and will cause some signal loss, reflection or attenuation, just as different layers of cladding having differing refractive indices cannot be fully matched by the gels or oils referred to above. Nevertheless, such techniques have yielded a variety of devices which meet most common needs and are entirely adequate for such purposes at the present state of the art in optical communications.
Some types of devices exhibit problems which are substantially less tractable. For example, as communication systems and instrumentation has become more complex, so-called splitters and star couplers are needed to couple an optical signal in one fiber to one or more other fibers. The comparable electrical function is readily accomplished by providing a low output impedance and current drive capability of a circuit providing input to a plurality of circuits having high input impedance. The quantification of the number of electrical circuits to which a signal in another circuit can be coupled is commonly referred to as fan-out.
However, in optical systems, such coupling requires some lateral component of light transmission (since coaxial coupling from one fiber to more than one fiber is topologically impossible) which the usual structure of optical fibers is designed to prevent. Therefore, the geometry of choice has become the so-called fused biconical taper (FBT) star coupler which provides several modifications of fiber geometry which allows coupling to occur.
An FBT star coupler, in its simplest form, typically comprises a plurality or bundle of fibers in which a relatively short length of the bundle is fused together and reduced in cross-section, thus taking on the form of two cones joined at the respective apices thereof which is the basis of the name. The formation of such a structure is typically performed at the present state of the art by a highly trained technician using sophisticated optical equipment. Commonly, while monitoring the process by injecting light into one (usually central) fiber or several fibers and monitoring the light output of other fibers, the bundle of fibers is heated to about 1700.degree. C. to cause fusing (but not viscous flow) of the fibers. The heated portion of the bundle is then stretched to reduce the diameter of the bundle and each fiber therein and twisted to cause compression between the fibers to thus form the biconical portion of the star coupler. At some point in this process, the coupling from one fiber to another will come within predetermined coupling specifications and the process is terminated.
Star couplers made in this fashion exhibit substantial differential variations in coupling efficiency to respective output fibers. Therefore, sophisticated and delicate correction techniques have been developed in order to effect partial correction of consistency of coupling efficiencies of star couplers in a post-production process to reclaim portions of manufacturing yield which would otherwise be lost. For example, one such technique involves breaking the star coupler between the conical sections, rotating one section by 180.degree. and rejoining the sections to cause averaging of differences in coupling efficiency.
In summary, star couplers must be made by an essentially manual process by skilled personnel and, at the present level of skill in the art, are not susceptible of automated manufacture since the sufficiency of coupler formation must be monitored during the manufacturing process. Even with highly skilled personnel, manufacturing yield is relatively low and post-production correction is delicate, slow and also requires highly trained personnel.